Hydrocarbon gas stream contains lighter components (e.g. hydrogen, nitrogen, etc.) methane, ethane and a substantial quantity of hydrocarbons of higher molecular weight, for example, propane, butane, pentane and often their unsaturated analogs. Recent changes in ethylene demand have created increased markets for ethylene and have created a need for more efficient processes which yield higher recovery levels of this product. In more recent times the use of cryogenic processes utilizing the principle of gas expansion through a mechanical device to produce power while simultaneously extracting heat from the system have been employed. The use of such equipment varies depending upon the pressure of the gas source, the composition of the gas and the desired end results. In the typical cryogenic expansion-type recovery processes used in the prior art, a gas stream under pressure is cooled by heat exchange with other streams of the process and/or external sources of cooling are employed such as refrigeration systems. As the gas is cooled, liquids are condensed, collected, and separated so as to thereby obtain desired hydrocarbons. The high pressure liquid feed is typically transferred to a demethanizer column after the pressure is adjusted to the operating pressure of the demethanizer. In such a fractionating column the liquid feed is fractionated to separate the residual methane and lighter components from the desired products of ethylene and heavier hydrocarbon components. In the ideal operation of such separation processes, the vapors, or light cut, leaving the process contain substantially all of the methane and lighter components found in the feed gas and substantially no ethylene and heavier hydrocarbon components remain. The bottom fraction, or heavy cut, leaving the demethanizer typically contains substantially all of the ethylene and heavier hydrocarbon components with very little methane or lighter components which are discharged in the fluid gas outlet from the demethanizer. A typical combined gas expansion and fractionation process for the separation of hydrocarbon gas stream comprising components ranging from nitrogen through C.sub.3 +hydrocarbons into a methane and lighter stream and an ethylene and heavier stream is exemplified by U.S. Pat. No. 4,895,584.
Pressure swing adsorption (PSA) provides an efficient and economical means for separating a multi-component gas stream containing at least two gases having different adsorption characteristics. The more strongly adsorbable gas can be an impurity which is removed from the less strongly adsorbable gas which is taken off as product; or, the more strongly adsorbable gas can be the desired product, which is separated from the less strongly adsorbable gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feed stream to produce a purified (99+%) hydrogen stream for a hydrocracking or other catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more strongly adsorbable gases, such as ethane, from a feedstream to produce an ethane-rich product.
In pressure swing adsorption, a multi component gas is typically fed to at least one of a plurality of adsorption zones at an elevated pressure effective to adsorb at least one component, while at least one other component passes through. At a defined time, the feedstream to the adsorber is terminated and the adsorption zone is depressurized by one or more cocurrent depressurization steps wherein pressure is reduced to a defined level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. Then, the adsorption zone is depressurized by a countercurrent depressurization step wherein the pressure on the adsorption zone is further reduced by withdrawing desorbed gas countercurrently to the direction of feedstream. Finally, the adsorption zone is purged and repressurized.
The combined gas stream produced during the countercurrent depressurization step and the purge step is typically referred to as the tail gas stream. The final stage of repressurization is typically performed by introducing a slipstream of product gas comprising the lightest gas component produced during the adsorption step. This final stage of repressurization is often referred to as product repressurization.
In multi-zone systems there are typically additional steps, and those noted above may be done in stages. U.S. Pat. Nos. 3,176,444 issued to Kiyonaga, 3,986,849 issued to Fuderer et al., and 3,430,418 and 3,703,068 both issued to Wagner, among others, describe multi-zone, adiabatic pressure swing adsorption systems employing both cocurrent and countercurrent depressurization, and the disclosures of these patents are incorporated by reference in their entireties.
Various classes of adsorbents are known to be suitable for use in PSA systems, the selection of which is dependent upon the feedstream components and other factors generally known to those skilled in the art. In general, suitable adsorbents include molecular sieves, silica gel, activated carbon and activated alumina. When PSA processes are used to purify hydrogen-containing streams, the hydrogen is essentially not adsorbed on the adsorbent. However, when purifying methane-containing streams, methane is often adsorbed on the adsorbent along with the impurity. The phenomenon is known in the PSA art as coadsorption.
Japanese Patent No. 1039163, issued Mar. 31, 1981 to Union Carbide Corp., discloses a process for the purification of methane by the removal of ethane from a methane-containing feedstream. The patent discloses a PSA process that employs the use of silica gel as the adsorbent. The patent discloses that the silica gel adsorbent provides (1) high differential loading for all impurities to be removed from the product methane, (2) good enrichment of impurities in the waste gas, and (3) ease of cleaning of the bed with low pressure purge gas. It is further stated that high differential loadings permit relatively small adsorption zones which are low in cost and which reduce frequency of desorption, and hence reduce the product loss associated therewith. Enrichment of impurities in the waste gas reflects the degree of separation achievable in the process and is important in order to reject the impurities with minimum loss of product component. Ease of cleaning (or desorption) permits a high purity methane product to be obtained with an economically small quantity of purge gas.
U.S. Pat. No. 5,013,334 issued to Maurer discloses a PSA process for the separation of ethane from a feedstream comprising mixtures of ethane and methane using a zeolite molecular sieve containing at least 20 equivalent percent of Zn cations and containing not more than 80 equivalent percent alkali metal cations wherein ethane and methane are adsorbed in an effluent stream enriched in methane relative to the feedstream is recovered. In a related application, U.S. Ser. No. 696,383, Maurer discloses a similar process using a calcium-Y zeolite molecular sieve adsorbent.
U.S. Pat. No. 4,769,047 issued to Dye discloses a process for the recovery of ethylene from the reactor effluent of the direct oxidation of ethylene to ethylene oxide. The ethylene is recovered from the vented light gas by first contacting the vented gas with an activated carbon adsorbent to adsorb C.sub.3 and higher hydrocarbons and subsequently by pressure swing adsorption with a zeolitic molecular sieve adsorbent to separate the ethylene from carbon dioxide.
The fluid catalytic cracking (FCC) process is a petroleum refining process for the conversion of straight-run atmospheric gas/oil, vacuum gas oils, certain atmospheric residues and heavy stocks recovered from other refinery operations into high-octane gasoline, light fuel oils and olefin-rich light gases. The FCC process typically contains a reactor and catalyst regenerator, a main fractionator, and a gas concentration section. A book entitled, "Handbook of Petroleum Refining Processes" edited by Robert A. Meyers, published by McGraw Hill Book Company, New York, 1986, describes the typical FCC process on pages 2-18 to 2-24 and particularly describes the operation of the gas concentration section of the FCC process on pages 2-21 to 2-22. The above pages are herein incorporated by reference. In the reactor section, the FCC feedstock is cracked in a fluidized bed of catalyst to produce an FCC reactor effluent containing hydrocarbons ranging from methane through the highest boiling component in the FCC feedstock. In addition, hydrogen and hydrogen sulfide are produced. The main fractionator separates the FCC reactor effluent into an overhead stream comprising gasoline and lighter components, and liquid product. The gasoline and lighter components are separated in the gas concentration section into FCC off gas, C.sub.3 -C.sub.4 's, and debutanized gasoline. The FCC off gas typically comprises hydrogen, carbon monoxide, nitrogen, methane, ethylene, ethane, and heavier components such as propylene, propane, butenes, butane and pentanes. Typically, the FCC off gas is used for fuel in the refinery. However, where the refinery is located in close proximity to an ethylene plant, the FCC off gas may be sent directly to the ethylene plant for the subsequent recovery of ethylene.
An ethylene plant is a very complex combination of reaction and gas recovery systems which produce ethylene by the pyrolysis or cracking of a wide range of hydrocarbon feedstocks. Typically, the hydrocarbon feedstocks, such as natural gas, naphtha or gas oil, are charged to a cracking zone in the presence of steam to produce a reactor effluent gas mixture. This reactor effluent gas mixture is subsequently separated into purified components through a complex sequence of cryogenic and fractionating steps. A typical ethylene separation section of an ethylene plant containing both cryogenic and fractionation steps to recover an ethylene product with a purity exceeding 99.5% ethylene is described in an article by V. Kaiser and M. Picciotti entitled, "Better Ethylene Separation Unit," appeared in HYDROCARBON PROCESSING MAGAZINE, November 1988, pages 57-61 and is herein incorporated by reference.
The existing cryogenic and fractionation system in the ethylene plant can recover the ethylene in the FCC off gas, but the penalties of this operation are significant. The high level of light components such as hydrogen, nitrogen, and methane, significantly raise the compression and refrigeration requirements in the ethylene plant for the incremental amount of ethylene recovered from the FCC off gas. Hence, it can be seen in view of the foregoing that recovering ethylene from FCC off gas is an expensive and complex process involving extensive compression and cryogenic fractionation to separate light gases such as hydrogen and methane from the ethylene. Processes are sought which enable the concentration and recovery of ethylene and heavier components from FCC off gas without expensive compression and cryogenic separation steps to remove the lighter components.